Despite growing opposition from airlines and industry unions to the U.S. Transportation Security Administration's new policy allowing passengers to carry small knives on planes, the agency's head today said he is not backing down.

"I have to make sure that TSA's focus is on those things that are most destructive to the aircraft," TSA administrator John Pistole told ABC News. "It is not pocket knives. It is those non-metallic improvised explosive devices, the bombs that are very small. They are concealable and they are well designed."

The new TSA policy, which will go into effect on April 25, will permit folding knives that do not lock and that have blades 2.36 inches or less in length and are less than 1/2-inch wide. Novelty-sized and toy bats less than 24 inches long, billiard cues, ski poles, hockey sticks, lacrosse sticks and two golf clubs will also now be allowed in carry-on luggage.

The move will conform to international rules that allow small knives and sporting goods. Razor blades, ice picks, scissors and box-cutters -- like those used by the 9/11 terrorists -- will still be banned. Full-sized baseball bats will also remain on the prohibited carry-on list.

The change, which Pistole announced on March 5, came after a TSA working group recommended that such items were not a security threat. He reiterated that liquid explosives, meaning improvised explosive device or I.E.Ds, remain the "greatest concern."

"It's these kinds of things that are easily concealable and they are simply not detected by walking through metal detectors," Pistole said. "We are trying to focus more on terrorist intent."

But the new policy has drawn massive criticism and concerns from three major U.S. airlines, some pilots, flight attendants, federal air marshals, insurance companies and politicians.

In a letter to Pistole dated March 12, American's senior vice president Will Ris wrote that the Fort Worth, Texas-based carrier agreed that it was "important" for the TSA to "regularly revisit its rules and regulations" and said American would "adhere fully" to TSA's policies, but added that he wanted to "voice American's concern" over the new change.

"We encourage the TSA to reassess its proposed revisions to the prohibited items list," Ris wrote.

Delta, which is the world's second-largest airline, was the first to speak out against the policy.

In his letter last week, Delta's CEO Richard Anderson said although the Atlanta-based carrier had a strong relationship with the TSA, he disagreed with the agency's recent decision and shared the "legitimate concerns" of flight attendants. Anderson also pointed out that small knives have been banned from commercial planes for the past 11 years.

US Airways CEO Doug Parker also urged Pistole to "reconsider" the policy, writing in a March 11 letter, "US Airways fully supports the continuous review and amendment of TSA policies. We also understand and support the risk-based assessment employed by the TSA. However, this review and policy amendment process is most effective when it is conducted in a collaborative way with airlines and their flight crews."

"In particular, seeking input before implementing a change in policy that might place our flight attendants' safety at risk would have provided a more thoughtful path to the desired outcome of secure and safe air travel."

World's Largest Pilot Union Supports TSA Policy Change

Flanked by flight attendants and pilots, Rep. Ed Markey, D-Mass., held a press conference at Boston's Logan Airport on Tuesday, in which he urged the TSA to abandon its new policy. If the TSA does not, Markey said he would introduce legislation to prevent knives from being allowed on planes, the Associated Press reported.

The Flight Attendants Union Coalition, which represents nearly 90,000 flight attendants, launched a wide campaign to kill the TSA policy change two days after it was announced. Flight attendants posted a petition on the White House's "We the People" website, asking the Obama administration to "tell the TSA to keep knives out of the cabin." The petition now has more than 26,200 signatures.

"Our nation's aviation system is the safest in the world thanks to multilayered security measures that include prohibition on many items that could pose a threat to the integrity of the aircraft cabin," the coalition, which is made up of five unions, said in a statement last week. "The continued ban on dangerous objects is an integral layer in aviation security and must remain in place."

Pistole told ABC News he met with the Flight Attendants Union Coalition today, and said pilots and flight crews were "valuable partners in layered security" to the TSA. When asked about the union's response to their discussion, he said, "we agreed to disagree."

Mike Karn, the president of the Coalition of Airline Pilots Association, which represents over 22,000 professional airline pilots at carriers including American Airlines and US Airways, said in a statement last week that the organization will "categorically reject a proposal to allow knives of any kind in the cabin."

However, the Air Line Pilots Association, the largest airline pilots union in the world, representing nearly 51,000 pilots, supports the TSA's new carry-on policy change.

"ALPA supports TSA efforts to streamline security and shift focus to individuals who intend to do harm. This will standardize TSA policy with the international community," the organization said in a statement. "Common sense risk-based security screening initiatives, like Known Crewmember and Pre-check, are the answer to protecting our nation's air transportation system. These TSA initiatives increase resources for screening so that they focus on the real security threats instead of objects."